Purification of benzene hexachloride



Patented Feb. 5, 1952 PURIFICATION OF BENZENE HEXACHLORIDE Bill M.Williams, Midland, Mich., assignor'to' The Dow Chemical Company,Midland, Mich., a corporation of Delaware No Drawing. ApplicationOctober 22, 1947, Serial No. 781,479

1 Claim.

This invention relates to a treatment of hexachlorocyclohexane, alsoknown as benzene hexachloride (CcHeCle), to alter the ratio of theisomers present and to obtain fractions greatly enriched with respect toparticular isomers. It relates in particular to a method for obtainingfractions of benzene hexachloride greatly enriched as to their contentof the gamma isomer and substantially free from irritating odor andcontaminating color.

The insecticidal activity of benzene hexachloride has been widelypublicized in recent years. It is known that this activity is centeredprincipally in the-gamma isomer, and that the alpha, beta, delta andepsilon isomers have relatively little insecticidal effect. The usualmethods of preparation of benzene hexachloride inevitably producemixtures of the alpha, beta and gamma isomers together with one or bothof the delta and epsilon isomers, and possibly others as yet unisolated.As first prepared, the alpha isomer is present in the greatestproportion while the amount of the beta isomer is commonly from to 20per cent and that of the gamma isomer from 2 to 12 per cent. The mixedproduct is not only less effective as an insecticidal agent than is thegamma isomer alone, but it usually has a strong odor which may be quitedisagreeable. That odor has been found to be absent from or at a minimumin the gamma isomer alone, and,

regardless of whether the odor is due to one or a mixture of the otherisomers or is due to common impurities, it is deemed desirable to removeit from those fractions of the product which are to be used forhousehold insecticidal purposes.

It is the principal object of the present invention to provide a methodfor altering the ratio of isomers present in a mixture of isomericbenzene hexachlorides. A particular object is to provide a process fortreating a mixture of isometric benzene hexachlorides to produce afraction greatly enriched as to the gamma isomer. A further object is toprovide such a process whereby the objectionable or disagreeable odormay be removed from the insecticidally useful fractions of benzenehexachloride. It is another object to make such process as simple aspossible, so that it may be carried out readily in standard industrialequipment.

In accordance with the present invention, there has been developed apractical process whereby the foregoing and related objects may beattained. This process comprises a selective solvent extraction, tominimize the amount of the beta isomer to be treated subsequently,followed by a vacuum distillation of the resulting mixture with anegligible beta isomer content. It has been found that, while areasonably complete separation of the various isomers is possible byusing a variety of solvents in a series of fractional crystallizations,such process is not as economical as might be desired. It has been foundas well that a mixture of benzene hexachloride isomers containing morethan about 5 per cent of the beta isomer cannot be fractionallydistilled to obtain a practical separation, since the extremely highmelting beta isomer causes mixtures containing it to sublime duringattempted distillation, and the sublimate tends to plug the apparatus,increasing the pressure therein and reducing both the rate ofdistillation and the degree of separation below practical limits. When,however, the amount of beta isomer is reduced to a value below 5 percent, and preferably below 4 per cent, the remaining mixture of isomersmay be subjected to fractional distillation at reduced pressure and itis found that such treatment results in a marked change in the ratios ofisomers present in the distilled fractions as compared with the ratio inthe original mixture.

The first step in the process may be carried out using benzene, carbontetrachloride, propylene chloride, or any of several other solvents todissolve preferentially the gamma, delta and epsilon isomers and part ofthe alpha isomer from the original mixture, leaving undissolved a partof the alpha isomer and nearly all of the beta isomer. Of the varioussolvents which may be used in this step, the most practical andconvenient one for commercial operation is benzene, since this materialis usually present in large amounts in the reaction mixture in whichcrude benzene hexachloride is first obtained by addition of chlorine tobenzene. Partial distillation of the excess benzene from such reactionmixture causes crystals rich in the beta isomer, and containing some ofthe alpha isomer, to separate from the initial solution. Similarly,treatment of solid crude benzene hexachloride with hot benzene, or othersolvent for the other isomers which is a poor solvent or a non-solventfor the beta isomer, results in a practical reduction or elimination ofthe beta isomer from the rest. The solution is separated from theinsoluble crystals, and is ready for the second step of the presentprocess.

The filtrate from the first step is heated to distill therefrom thevolatile solvent present. The high boiling residue may be crystallizedand then distilled, or it may be subjected directly to vacuumdistillation, and the distillate may be separated into fractions ofincreasing boiling points.

hereas the crude mixture and that resulti am the solvent treatment toremove the beta )mer are both characterized by a strong and rite-tingodor, the distillate is practically odorss with the exception of thelowest boiling fracin thereof. Similarly, while the crude products esomewhat colored, the vacuum distillates are 'lorless and solidify oncooling to form pure aite crystals. Most of the original color isconntrated in the still residue. Analysis of the icuum distillates bycomparison of their infrad absorption curves with those of known pure:imers shows the distillation to have effected substantial separation ofthe isomers and to we produced fractions greatly enriched as to Leinsecticidally active gamma isomer. The eld of purified product is highand the process much more economical than the relatively inef- :ientprocesses depending entirely on fractional main crop of tan coloredcrystals separated which, I

whenidried, contained and had a melting range from 60 to 100 C. Aportion of this material Weighing 463 grams was taken for distillation.When heated under vacuum, 74 grams of low boiling solvent was removed asa first fraction, leaving 389 grams of \\benzene hexachloride to bedistilled. Distillation was effected at 5 millimeters (Hg) absolutepressure with a reflux ratio of to 1. The weights and analyses of thefractions obtained are given in the following table. Beforedistillation, the crystals had the characteristic odor of crudeystallization. 20 benzene hexachloride.

Boiling Weight Infra- Red Analysis Fraction No. 0 Range,

0J5 Grams Per Cent Alpha Beta Gamma Delta Epsilon l 73-150 74 16. 0 0 00 0 2. 150-151 80. 6 17. 4 57 0 12 28 1 3 151-152 44. 3 9. 6 46 0 26 244 4 152-154 41. 9 9. 1 0 25 5 5. 154-156 55. 5 12. 0 20 0 43 28 6 6.l56-l57 48. 9 10.6 10 0 42 35 7 7. 157-159 61.8 13. 3 0 0 40 13 Residue56.0 12. 0 0 33 6 24 12 Totals. 463. 0 100.0

1 Trace.

The following examples illustrate the practice the invention:

Example 1 A crude benzene hexachloride, having a dark rown color andcontaining the following isoieric forms of the compound:

Per cent lpha-isomer' 55 eta-isomer 12 amma-isomer l5 elta-isomer 9ndetermined (solvent) 9 It is seen that by careful vacuum distillationit is possible to concentrate the initially small amount of the beta.isomer in the undistilled residue, as well as to obtain distilledfractions ranging from those which are high in the alpha isomer to thosewhich have little or none of the alpha or beta isomers and areconsiderably enriched in the gamma isomer. Fractions 2 through 7 werelight colored materials which solidified as masses of white crystalsessentially free from odor and having none of the characteristicirritating odor of the crude product. The residue was brown in color.

Example 2 Some of the benzene hexachloride tan colored crystals whichwere used as the material to be distilled in Example 1 were givenfurther successive treatments with isopropyl alcohol and then withtrichloroethylene, with intervening crystallization. Theso-recrystallized material was analyzed and found to contain 34 per centof the alpha, none of the beta, and 63 per cent of the gamma isomer.After heating 197 grams of this material under vacuum to removeremaining solvent, 182 grams were left for fractional distillation. Theresults are tabulated as before.

Boning Weight Infra- Red Analysis Fraction No. 0 Range,

Grams Per Cent Alpha Beta Gamma Delta Epsilon s. 6 4.8 is o 73 0 sTotals. 182. 0 100. 0

This distillation ran smoothly with no evidence of sublimation orplugging of the apparatus, and

it is seen that the gamma. isomer can be freed mer are not limited tofractional distillation.

While the latter results in concentration of the alpha isomer in lowerboiling fractions and of the gamma, delta and epsilon isomers, whenpment, in the higher boiling fractions, a straight vacuum distillationin which only a small forefraction and a still residue are separatedfrom the main distillate is equally eilective in producing asubstantially odorless and colorless product.

The distillations specifically reported in the examples, were carriedout in glass apparatus but the invention, is equally applicable todistillations carried out in-iron, nickel or other industrialldistillltion equipment, under reduced pressures of the order'oi'fll to 100millimeters- The method which comprises suspending ;a mixture ofisomeric forms of benzene, hexachlojride which cannot befractionallydistilled to ob tain a practical separation, in a liquid which is abetter solvent for the gamma isomer than for the beta isomer, separatingtherefrom a crop of crystals rich in the beta isomer, recovering fromthe liquor a mixture of isomeric forms of benzene hexachloridecontaining less than per cent of 6 the beta isomer, fractionallydistilling the mixture at a pressure of from 1 to millimeters ofmercury, and recovering the lower boiling fractions enriched as to thealpha isomer and the higher boiling fractions enriched as to the gammaisomer, said principal fractions being improved both in odor and coloras compared with the mixture subjected to distillation.

BILL M. WILLIAMS.

' REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS OTHER REFERENCES Morton, Laboratory Technique inOrganic Chemistry, published 1938 by McGraw-Hill Book Company, New York,New York, page 99.

Slade, The Gamma-Isomer of Hexachloro Cyclohexane," Chemistry andIndustry, Oct. 13, 1945, pages 314-319.

